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Axelson ®
Pilot-Operated Safety Relief ValvesSeries H
In the mid 1940s, a new relief valve actuated with a pilot rather than a spring set a new standardfor pressure control. The valve, manufactured over the years by companies like Garrett, USI andAxelson, is now manufactured and sold by NuFlo Measurement Systems under the Axelson brand.
NuFlo's Axelson® pilot-operated pressure relief valves are an example of a tradition of advanceddesign and high-quality manufacturing abilities. These valves offer advantages not found in otherrelief valves-spring- or pilot-operated.
The name Axelson has stood for ingenuity, quality and service for more than 100 years and NuFlo iscommitted to preserving that tradition for years to come.
Full Flow Capacity
Backflow Protection
Simple Maintenance
Non-Flowing Pilots
Coated Bodies
Easier Handling
Type HF This is a full lift, pop-action valve with a fixed (5% to 7%)blowdown using a single non-flowing pilot. This type of valve isdesigned for gas and/or vapor service and is offered in sizes of 1" through 8" with operating pressures ranging from 20 psig to6000 psig.
Valve Types
Full Flow Capacity In addition to the various API orifice sizes, NuFlo offers valveswith non-standard API orifice sizes for maximum flow capacity.Many times this allows for the use of smaller size valves at a costsavings to the customer.
Backflow ProtectionThe unique NuFlo split piston (optional) is designed to eliminatethe effect of pressure in the discharge system back-flowing into arelief valve on installations where several valves discharge into acommon manifold.
Simple MaintenanceAll maintenance, including changing the valve seat, can beperformed using ordinary hand tools without removing the valvefrom the installation.
Non-Flowing PilotsNuFlo non-flowing pilots reduce the problems of “freeze-up”caused by the pressure drop through the flowing-type pilots.
Coated BodiesThe bodies and piston housings of all NuFlo pilot-operated safetyrelief valves are internally and externally coated with Xylan® forcorrosion protection and lubricity.
Easier HandlingA lifting eye is conveniently located on the center of the valve forease of handling during installation or removal.
Advantages
Soft or Hard Seat Seals Soft or hard seat seals are available for a variety of serviceconditions and applications. Soft seat seals are recommended fordischarge set pressures of 25 psi to 1500 psi. Hard seat seals arerecommended for discharge set pressures above 1500 psi.
Variable Flange DimensionsFlange dimensions can be modified on special order to fit mostexisting installations. This permits NuFlo pilot-operated safetyrelief valves to be used as replacements for older spring-loadedvalves which may not conform to new safety standards.
In-Service Test Kit This optional feature allows checking or changing the pilot setpressure in the field with the valve in service.
Manual Blowdown This optional device, which allows manual blowdown of thesystem, can also be controlled from a remote location.
Direct or Remote Control Depending on the application, the operation of the NuFlo pilot-operated safety relief valve may be controlled directly from thepoint of installation or remotely.
No “Simmer”NuFlo pilot-operated safety relief valves are designed to eliminate"simmer" at the valve seat. They do not require "percentaccumulation" or over-pressure to operate.
Special FlangesNuFlo pilot-operated safety relief valves can be supplied withspecial flanges such as Graylock, Taper-Lok, Lenz, etc.
Features
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Type HL This is a modulating valve with a fixed (3% to 5%) blowdownusing a single non-flowing pilot. This valve is designed for gas,vapor and/or liquid service and is offered in sizes of 1" through 8" with operating pressures ranging from 15 psig to1500 psig.
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Advantages and Features
Easy and economical to maintainAll maintenance can be performed without removing valve from line
YES YES NO
Replaceable soft seatSaves costly lapping of valve seat
YES YES YES
Operates without simmer at valve seatCan be set close to system operating pressure. Unaffected by vibration of pulsation
YES YES NO
Block and bleed pilot as standardReduces freeze-ups caused by pressure drop through flowing type pilots
YES YES NO
Accurate setting with small volume of pressureTest fixture available for fast accurate setting
YES YES NO
Backflow protectionPrevents flow of gas back through valve when working on line
YES YES NO
Combines functions of blowdown and safety valvesSaves cost of additional valves and piping
YES YES NO
Higher capacity per valve size YES YES NO
Field test of pilot set pressureSet pressure can be checked or changed with valve still in service NO YES NO
Can be used with solenoid valveFor electric or pneumatic interface
YES YES NO
Coated internally and externallyBodies and piston housings. Xylan coated for corrosion protection and lubricity
YES YES NO
Balanced pilotAllows venting into discharge system without effect of back pressure
NO YES NO
Advantages and FeaturesNuFlo Pilot Operated Competitive Spring
Loaded ValveHF HL
NuFlo pilot-operated safety relief valves operate on the principleof unequal areas exposed to the same pressure. When the reliefvalve is closed, system pressure pushes upwards against thepiston seat seal on an area equal to the inside diameter of theseat. Simultaneously, the same system pressure passes throughthe pilot, exerting a downward force on the piston acting on anarea approximately 50% greater than the inside diameter of theseat. The resulting differential force holds the valve tightly closed.As the system pressure rises, the force against the piston seal
increases. Then, when the system pressure reaches the reliefvalve discharge set pressure, the pilot cuts off system pressureand opens the top of the piston to vent pressure. As the pressureabove the piston is relieved, the relief valve opens, dischargingline pressure.
When the predetermined blowdown pressure is reached, the pilotshuts off the exhaust and re-opens the flow of system pressure tothe top of the piston, effectively closing the relief valve.
Valve Operation
A pilot-operated pressure relief valve, according to the 1992ASME Code Section VIII, Division 1, Section UG-126, is a pressurerelief valve in which the major relieving device is combined with,and is controlled by, a self-actuated auxiliary pressure relief valve.NuFlo pilot-operated pressure relief valves are designed to be
used wherever there is a need to exhaust the overpressure volumeof gas, vapor and/or liquid. Applications include oil and gasproduction systems, compressor stations, gas transmission(pipelines) facilities, storage systems, distribution systems and inall types of processing plants.
Application
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Relief Valve OpenWhen system pressure reaches the set point, thepilot piston is lifted off the valve seat. Theblowdown seat seals off incoming line pressure,causing the exhaust port to open and bleedpressure from the relief valve piston cavity.Decreasing pressure on the top of the relief valvepiston allows the valve to open, relieving systemoverpressure. As system pressure drops below theblowdown reset point, the blowdown seat opens,reseating the pilot piston, which causes theexhaust port to close. System pressure re-entersthe relief valve piston cavity, closing the reliefvalve.
Right: Type HF relief valve (relieving position)
Relief Valve ClosedAt below “set point”, the normally opencombination pilot allows system pressure to enterthe piston housing cavity of the relief valve on topof the free-floating piston. The top of the reliefvalve piston has a larger area than the valve seatwhere the piston seals. Equal pressure at bothends of the piston creates a differential downwardforce, which holds the piston tightly closed on thevalve seat.
Below: Type HF relief valve (closed position)
Operation Type HF
Type HF Relief Valve (Fixed Blowdown 5-7%)
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Relief Valve OpenWhen system pressure reaches the set point, thepilot piston forces the pilot stem upward bycompressing the pilot valve spring. This movementof the stem simultaneously blocks the systempressure passageway through the pilot andcommences the bleeding of pressure from therelief valve piston housing cavity. Decreasingpressure on the top of the relief valve pistonallows system allows the valve to open, relievingsystem overpressure. As system pressure dropsbelow the blowdown reset point, system pressurere-enters the relief valve piston cavity, closing therelief valve.
Right: Type HL relief valve (relieving position)
Relief Valve ClosedAt below “set point,” the normally open Type HLpilot allows system pressure to enter the pistonhousing cavity of the relief valve on top of the free-floating piston. The top of the relief valve pistonhas a larger area than the valve seat where thepiston seals. Equal pressure at both ends of thepiston creates a differential downward force,which holds the piston tightly closed on the valveseat.
Below: Type HL relief valve (closed position)
Operation Type HL
Type HL Relief Valve
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Type HL
Typical Direct Hook-Up with In-Service Test Valve(Gas Service Only)1. Connect pressure hose from nitrogen bottle or hydraulic
hand pump to test port of header block.
2. Close vent valve "A".
3. Slowly open block valve "B", or operate hydraulic hand pump, permitting test pressure to increase to valve set point.
4. Observe set pressure on test gauge and record.
5. Close valve "B", or release pressure on hand pump.
6. Open vent valve "A".
7. Disconnect pressure hose from test port.
NOTE: For additional information on In-Service testing, refer to theservice manual or technical data sheets for this product.
All NuFlo pilot-operated pressure relief valves may be orderedwith an In-Service Test Kit. The procedures for the use of this In-Service Kit are shown below.
Type HF In-Service Testing of Pilot Set PressureCAUTION: Never use oxygen as a pressure source.
Pressure from a cylinder of nitrogen or some other pressure source(NOT OXYGEN) may be used to check the setting or to reset thepressure at which the relief valve will operate.
1. Connect pressure hose from nitrogen bottle to field testvalve "A".
2. Close vent valve "B".
3. Open field test valve "A".
4. Slowly open block valve "C" permitting test pressure toincrease to valve set point.
5. Observe set pressure on test gauge and record.
6. Close valves "A" and "C".
7. Open vent valve "B".
8. Disconnect pressure hose from field test valve "A".
NOTE: For additional information on In-Service testing, refer tothe service manual or technical data sheets for thisproduct.
When a relief valve is not equipped with the In-Service Test Kit, aRelief and Blowdown test fixture can be used to check or changevalve set and blowdown pressures in field shops. Only the pilotsneed to be removed from the relief valve and it is not necessary toremove the valve itself from the installation.
The operation of this portable test fixture is simple andconvenient.
CAUTION: Never use oxygen as a pressure source.
Pressure from a cylinder of nitrogen or some other pressure source(NOT OXYGEN) may be used to check the setting or to reset thepressure at which the relief valve will operate.
Special training is not required and complete instructions arefurnished with each fixture.
The test fixture may be orderedwith optional needle valves andadapters for any type of pilot.
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The Type HL pilot is a single-control pilot with a fixed blowdownfor controlling the opening and closing of the relief valve. Openingand closing pressures are determined by the force of a pilotcontrol spring. System pressure is applied to the pilot controlpiston and also to the pilot inlet "Hi" port. The lifting forceproduced by the pressure on the control piston is reacted by theopposing force of the pilot spring. When the spring force isgreater than the pressure force of the control piston, systempressure is communicated through the inlet "Hi" port of the pilotto the top of the relief valve piston. Since the area on top of therelief valve piston is greater than the seat area, the valve is heldin the closed position. As pressure increases above the set point,the force of the control piston becomes greater than the reactingspring force. This unbalanced condition shifts the pilot stemupward, blocking the pressure coming into the inlet “Hi” port and
allowing the pressureabove the relief valvepiston to bleed off. Asthe pressure force,(which holds the pistonon the seat) decreases,the relief valve opens. If system pressurecontinues to rise, thepiston lifts fully andremains open untilsystem pressure isreduced sufficiently forthe pilot spring to shiftthe pilot into its flowingposition.
Pilot ConstructionThe Type HF pilot is a single combination control with a fixedblowdown for controlling relief valve opening and closingpressure set points. The opening set pressure is determined by theforce of a control spring, which holds the relief control section ofthe valve closed. When system pressure acting on the reliefcontrol valve seat area equals the spring force, the relief controlopens, and the blowdown control section closes, blocking systempressure from passing into the chamber above the main valvepiston. As the relief control opens, the pressure underneath thecontrol seat is exposed to a larger pressure area which provides“snap” action of the control pilot to quickly reduce pressure in thepiston dome. Thispressure reductioncauses the main valvepiston to lift, relievingsystem pressure.
After the systempressure is reduced to apoint whereby thecontrol valve springforces the blowdowncontrol ball to unseat,the relief control valvecloses, and the openblowdown control valveallows system pressureto re-enter the pistondome, forcing the mainpiston down to a closedvalve position.
Split Piston SeparatedThe split piston isrecommended for installationswhere a manifold dischargesystem serves a number ofrelief valves. If a manifoldedrelief valve is out of servicewhen one or more other valvesexhausts into the commondischarge system, the splitpiston separates and preventsbackflow through the out-of-service valve.
Normal Piston PositionAll NuFlo pilot-operated pressurerelief valves can beordered with theexclusive NuFlo splitpiston. This piston isdesigned to eliminatethe effect of back-pressure imposed onthe discharge side ofthe valve.
Function of the Split Piston
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Installation Schematics
Types HF
Type HL
Direct HookupRelief valves configured for directoperation are equipped with aninternal pressure pickup tube(stinger) in the throat of the valveinlet. They are factory shipped asself-contained assemblies, readyfor installation.
Remote HookupRelief valves configured forremote operation do not containan integral pressure pickupconnection. Pressure is sensedfrom a remote point on thevessel/process line through asingle tube connected to theheader block located on thevalve.
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Orifice Selection
Available Orifice Sizes for Type HF and HL Pilot-Operated Relief Valves Valve Coefficient: 0.859 (gas), 0.674 (liquid)
Trims Available(Materials of Construction)
Orifice Area (Sq. In.)
0.1100.1960.307
0.6520.785
0.1100.1960.307
0.7851.2871.6331.767
0.5030.785
1.6331.767
0.5030.7851.287
Valve Size Outlet
1.838
3" x 4" Single
4" x 6" Single/Dual
6" x 8" Single/Dual
8" x 8" Dual
8" x 10" Single/Dual
JKLMNP3"
LMNP4"
QRT6"
QRT7"
7-1/2"
1.2871.8382.8533.6004.3406.3807.068
2.8533.6004.3406.38012.566
11.04516.00026.00028.270
11.04516.00026.00038.48444.178
2.776
Orifice
DEF
0.5031" X 2" Single
GGX1"
DEF
0.5031-1/2" x 2" Single
GHJ
JX1-1/2"
GH
1.2871-1/2" x 3" Single JJX
1-1/2"
GHJ
2" x 3" Single 1.633JXK
KX2" 3.141
Orifice Area (Sq. In.)Valve Size Outlet Orifice
QRT7"
7-1/2"
11.04516.00026.00038.48444.178
NOTES: (1) A-216-Gr WCB (2) A-351-Gr-CF8M (3) HRc 22 maximum
Standard Stainless Steel Full Stainless Steel NACE
Body
Piston Housing
Piston
Valve Seat
Orifice
HF
CS (1)
CS (1)
AL
316 SS
316 SS
HL
CS (1)
CS (1)
AL
316 SS
316 SS
HF
CS (1)
CS (1)
316 SS
316 SS
316 SS
HL
CS (1)
CS (1)
316 SS
316 SS
316 SS
HF
SS (2)
SS (2)
316 SS
316 SS
316 SS
HL
SS (2)
SS (2)
316 SS
316 SS
316 SS
HF
CS (1)(3)
CS (1)(3)
316 SS
316 SS
316 SS
HL
CS (1)(3)
CS (1)(3)
316 SS
316 SS
316 SS
Header Block
Pilot Valve
CS
316 SS
316 SS
316 SS
CS
316 SS
316 SS
316 SS
316 SS
316 SS
316 SS
316 SS
CS (3)
316 SS
316 SS
316 SS
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Soft Seat Service
Continuous Temperature, °F Minimum Pressure, psi Maximum Pressure, psiMaterial
Max. Min. Pilot Main Pilot Main
Buna-N 275 -65 15 15 6000 1500
Viton 400 -65 15 15 6000 1500
Teflon 400 -423 - 60 - 1500
Peek 480 -423 - 1500 - 3000
S.S. - - - 1500 - 6000
HF1. Type of valve.
2. Inlet pipe size of valve.
3. Rating of inlet and outlet flanges: RF, RTJ or other.
4. Type of seal (hard or soft).
5. Set pressure (pressure at which valve will open).
6. Manual blowdown valve (optional).
7. In-service test unit (optional).
8. Construction and trim (unless otherwise specified, standardconstruction and trim will be furnished).
9. Characteristics of gas being controlled.
10. Specific gravity or molecular weight of gas.
11. Temperature of gas.
12. Back pressure (pressure on outlet).
134. Specify any special markings or identification.
HL1. Type of valve (gas or liquid service).
2. Inlet pipe size of valve.
3. Rating of inlet and outlet flanges: RF, RTJ or other.
4. Type of seal (hard or soft).
5. Set pressure (pressure at which valve will open).
6. Manual blowdown valve (optional).
7. In-service test unit (optional).
8. Construction and trim (unless otherwise specified, standardconstruction and trim will be furnished).
9. Characteristics of gas and/or chemical components ofproducts in system being controlled.
10. Specific gravity or molecular weight of gas and/or liquids.
11. Operating temperature of product.
12. Back pressure (pressure on outlet), if any.
13. Specify any special markings or identification.
NOTE: When ordering spare parts, please give valve serial numbers when available. For part numbers, refer to data sheets, whichare available on request.
Warnings1. The inside diameter (ID) of inlet piping (and block valve, if
installed) MUST equal or exceed the ID of the relief valveinlet. Any flow restriction in the inlet piping or block valvecan cause malfunctioning of the relief valve and willadversely affect the relieving capacity. Block valve should belocked in the OPEN position.
2. Risers longer than three pipe diameters should be at leastone pipe size larger than the relief valve.
3. Exhaust (outlet) must be directed away from any area thatmay be occupied and away from any nearby structures.Vertical exhaust is preferred.
4. Valve must be properly supported to withstand the reactivethrust created by the exhaust flow when the exhaust isvented to atmosphere, especially for side discharge valves.
5. Valves installed on long risers or with any restrictions in theinlet piping or any valve being used on liquid service shouldbe installed with a remote sensing line. Introduce sensingpressure as slow as possible to pilot to ensure piston domeis completely charged.
6. Care should be taken not to install a valve in a piping systemthat has the same natural frequency as the valve. Riserlength should be kept to a minimum to avoid major vibrationproblems. Contact NuFlo Measurement Systems forassistance.
Ordering Information
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For representation in your area:
NuFlo Measurement SystemsA NuFlo Technologies Company
USA: Houston, TX • Corpus Christi, TX • Longview, TX • Odessa, TX • Duncan, OKShreveport, LA • Houma, LA • Lafayette, LA • Laurel, MS • Bakersfield, CA Saginaw, MI • Casper, WY • Broomfield, CO • Dallas, TX • Tulsa, OK
Canada: Calgary, AB • Edmonton, AB
International: Jakarta, Indonesia • Aberdeen, Scotland • Bognor Regis, UK Dubai, UAE • Hassi Messaoud, Algeria • Singapore
North America:
Singapore:
Bognor Regis, UK:
www.nuflotech.com • HOUSTON HEAD OFFICE: 281.582.9500
Document NF 00033 02 0604Note: NuFlo Measurement Systems manufactures and sells Axelson® under license from Halliburton. Xylan® is a registered trademark of Whitford Corporation
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